1. Field of the Disclosure
The present disclosure relates to an organic electroluminescent device.
2. Discussion of the Related Art
An organic electroluminescent device is one of flat panel displays and has advantages of high luminance and low driving voltage. Further, since it is self-luminescent type, it has high contrast ratio, thin profile, no limit to viewing angle, stability at low temperature, easy displaying images due to response speed of about several microseconds, and simple fabrication and design of driving circuit due to low driving voltage of DC 5V to 15V.
Accordingly, the organic electroluminescent device is used various IT devices such as TV, monitor and cell phones.
The organic electroluminescent device includes an array device and an organic light emitting diode. The array device includes a switching thin film transistor connected to a gate line and a data line, and a driving thin film transistor connected to the organic light emitting diode. The organic light emitting diode includes a first electrode connected to the driving thin film transistor, an organic light emitting layer and a second electrode.
A light emitted from the organic light emitting diode travels toward the first electrode or the second electrode to display images. In light of aperture ratio, a top emission type of organic electroluminescent device is used that emits light toward the second electrode.
The organic light emitting layer of the organic electroluminescent device as above is formed with a thermal deposition method that uses a shadow mask made of metal material.
FIG. 1 is a schematic view illustrating a thermal deposition method using a shadow mask according to the related art.
An organic light emitting material 51 in powder form is located in a deposition device 50 having a heating means. In a chamber in a vacuum, when the heating means is operated and heat the deposition device 50, the heat is transferred to the organic light emitting material 51 and the organic light emitting material 51 is sublimated. The organic light emitting material gas 52 is emitted through an outlet of the deposition device 50, and is selectively deposited on the substrate 70 through a shadow mask 30 having opening portions OA1 and OA2 over the outlet of the deposition device 50 to form an organic light emitting layer on a substrate 70
Regarding fabrication of the shadow mask 30, a metal plate are patterned at both of a top surface and a bottom surface in a mask process, for example, including deposition of photoresist, light exposure, developing and etching to form the opening portions OA1 and OA2, and a shielding portion SA located between the adjacent opening portions OA1 and OA2. The opening portion OA1 at the top surface area has an area different from that of the opening portion OA2 at the bottom surface.
The patterning process to form the shadow mask 30 is relevant to an etching rate of the metal material of the shadow mask 30 and a thickness t of the shadow mask 30. In case of etching the shadow mask 30 in one direction, size difference of each of the opening portions OA1 and OA2 occurs much, and error due to area difference between the opening portions OA1 and OA2 for each location occurs much. To prevent this, the simultaneous etching for the top surface and the bottom surface are performed.
Accordingly, the area of the opening portions OA1 and OA2, and in more detail, the width (that is a width of the opening portion at the surface facing the substrate 70) is required to be at least 32 um in order that the width is in the error range permissible in fabrication.
In case that the width A of the opening portion facing the substrate 70 is less than 32 um, the opening portion OA2 at the bottom surface of the shadow mask 30 has a greater area if the thickness t of the shadow mask 30 is constant (e.g., t=40 um). In this case, a portion between the adjacent opening portions OA2 i.e., a width of a rib portion is very small, and this causes reduction of hardness of the shadow mask 30, and thus when the shadow mask 30 sags, shape deformation of the opening portions occurs.
The reduction of the thickness t of the shadow mask can solve this problem. However, the thickness of the shadow mask made of metal material is usually limited to 40 um, and a metal plate less than 40 um is not produced and thus fabricating the shadow mask having such the thickness is impossible. Even though such the thickness is allowed, a bearing power between the opening portions OA1 and OA2 become less and thus sagging becomes more.
Moreover, when the organic light emitting material is thermally deposited on the substrate 70 using the shadow mask 30, the material is spread in deposition process due to shadow effect and etc. In other words, the organic light emitting pattern is formed beyond the opening portions of the shadow mask. Further, location change of the shadow mask 30 and pitch change of the opening portions OA1 and OA2 of the shadow mask 30 occurs in deposition process. Considering such the various factors, a bank (not shown), which is formed at a boundary portion between adjacent pixel regions, is required to have a width of at least 12 um. However, forming the bank having at least 12 um causes increase of the boundary portion, and thus there is a limit to a resolution of the display device.
Moreover, when forming large-sized display device having 10 inch or more, the area of the shadow mask 30 is required to be increased, and in this case, the shadow mask, particularly a center portion thereof sags much due to increase of weight, and formation error of the organic light emitting layer occurs much.
Accordingly, fabricating the organic electroluminescent display having the large size of at least 10 inch and having high resolution of at least 250 PPI (pixels per inch) using the shadow mask 30 is difficult.